(In reply to Sam Penrose from comment #4)
> Do we have an ETA on this?
I'm planning to kick off the backfill proper once we know that it won't overwhelm the system. The PR above sets it to run a max of 10 days of backfill at once, so that should leave some capacity for all the other jobs.
Once the jobs are all running, I expect it will take a few days to complete.

After a fair amount of debugging with Airflow's backfill capability, a backfill-specific version of the DAG is now running, namely the one added in the PR from comment 8.
It will be backfilled for 180 days of history (back to 2016-03-12) per the usual retention period.